Recently, effective utilization of water resources is required because of industrial development. Therefore, reuse of wastewater such as industrial effluent and the like is very important. In order to achieve the purpose, purification of water, namely, to separate other substances from water is necessary.
As a method for separating other substances from liquid, various methods have been known. Examples of the methods include membrane separation, centrifugation, activated carbon adsorption, ozone treatment, flocculation, and removal of suspended matter with a particular adsorbent. By using such a method, chemical substances largely affecting environment such as phosphorus and nitrogen contained in water can be removed, and oil and clay dispersed in water can be removed.
Among these methods, the membrane separation is one most generally used. However, when oil dispersed in water is removed, there is a problem that the oil tends to clog fine pores of the membrane to result in short life. Therefore, in many cases, the membrane separation is not appropriate for removal of oil in water. Thus, as a method for removing oil such as heavy oil from water include, for example, a method of gathering heavy oil floating on water surface with an oil fence set on water by making use of levitation property of heavy oil and suctioning to recover from the water surface, or a method of spreading hydrophobic matters having adsorbing property to heavy oil on water and adsorbing to recover heavy oil.
From the viewpoint of this, recently, an approach of immersing an oil adsorbent in water where oil is dispersed and adsorbing to remove the oil from water with the oil adsorbent has been tried. For example, JP-A 60-97087 (KOKAI) discloses a technique of adsorbing to remove oil from water using an oil adsorbent comprising magnetic particles covered with organic substances such as resins on the surface thereof. However, according to the method, due to low dispersibility in water, the functional particles tend to settle in or float on the water surface; thus, the oil could not be efficiently adsorbed and removed.
Further, JP-A 07-102238 (KOKAI) discloses a method of adsorbing oil by using adsorptive polymer as an oil adsorbent having a hydrophilic block and a lipophilic block, and then removing the adsorptive polymer from water. However, such a method has a problem that not only it takes much labor to separate the adsorptive polymer and water but also the adsorptive polymer to which the oil is adsorbed is made softened to deteriorate the mechanical strength, resulting in poor workability.